Artificial reefs
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Contents
Introduction
An artificial reef (AR) can be defined as any solid man-made structure which has been submerged in the natural environment (Bohnsack, 1989[1]). They may be purposely placed to alter local hydrodynamics (coastal defense ARs) or this effect may be incidental (ship wrecks, offshore windmills, oil rigs etc.). Regardless of their construction or purpose, artificial reefs generally involve the introduction of a hard substrate to a soft bottom environment, altering both the abiotic as the biotic properties of the environment. Coastal defense artificial reefs are therefore often constructed as multipurpose structures, i.e. besides the main function to defend the coast from erosion they also maximise secondary objectives such as to improve spot surfability, to stabilize nourishment material and to create marine parks of increased biodiversity. Since different aims, materials and configurations as well as site dependent factors are to be considered, design criteria for these structures are not easily defined a priori.
Materials
Stones
Stones are used especially for rubble mound structures whose hydraulic stability is verified through the formulae available in literature for low-crested structures (Burcharth et al., 2007 [2]).Un-corrected design of the structure (and thus a wrong selection of stone size) causes a low efficiency of the AR. Structure reshaping induced by wave breaking over the reef may occur and produce instability, with stones rolling down from the structure. Stones are relatively cheaper than other materials but can be more dangerous for surfing safety.
Geobags, geotextile sand containers and geotubes
Geotubes consist of sacks made by geotextile that are filled with sand or gravel. They are characterized by a length and diameter of 20 and 3 m respectively and are particularly recommended for deep bottoms (Matteotti et al., 2003[3]). Geotextile sand containers (GSCs) and geobags are very similar to geotubes, although their dimensions are smaller than that ones and thus are usually adopted at lower depths. The cost of a reef made of GSCs is usually greater than in case of other materials because it’s more difficult the placement. GSCs and geobags are often employed as revetments or as seaward slopes of nearshore structures. It has to be highlighted that exhaustive analysis regarding element stability are not available and therefore it is need to combine numerical and physical models with practice and experience in the field to establish the real stability of the GSC.
Reef units
Reef units of different shapes and constructive characteristics can be used to produce ARs. Their macro-roughness induces the dumping of the incident wave agitation above the reef, creating local turbulence and vortices. Usually they are made by steel, reinforced or pre-stressed concrete, fibreglass or a variety of composite materials. These units are produced individually on shore and transported to the staging area. Here they could be combined in a variety of configurations thus allowing adjustment to local conditions and needs. Most of the units are designed to provide nursery areas for fish, to permit a good water recirculation, promote current deflections and marine colonization (O’Leary, 2001[4]). Some examples are shown in Figure 1.
Figure 1.e shows the so-called tecnoreef modules. Constructed from concrete-based natural products, they pose little impact on the ecosystem. Each module consists of reinforced octagonal plates, which are characterized by holes (usually four) having a quarter of a circle shape. The modules are assembled together to create a sort of pyramid that usually consists of three octagonal plates, the so called “base system”. The structure basis is made larger than the crest thus ensuring the resistance and stability to cross-shore currents and drag forces. They globally reduce cross-shore currents due to friction and turbulence The consequent deposition of transported sediments within and around the pyramids is increased by the differently inclined facets on the external and internal surfaces of the modules. These irregularities facilitate localized microcurrents and create continuous circular currents (spheres of water) which release their energy upwards within each element. The constant circulation and exchange of water allows the influx of nutrients and permanent occupation by flora and fauna.
Equally note-worthy are the reef ball elements shown in Figure 1.f. These have an hemispherical shape (Harris, 1995[5]) and are typically 1-2 m high and 2 m wide. A reef ball is made by polyester with glass reinforcement and is thus very resistant to corrosion. The reef ball contains a buoy that allows it to float during construction phase and that is deflated when the element is definitively placed. They are characterised by high porosity (40-50%), roughness and many cavities or holes which have a double action. First off, the roughness and holes increase the structure complexity which assures a quick colonisations by marine species and aquatic plants (Pilarczyk, 2003[6]). Secondly, these structures are specifically designed to generate turbulence, vortices and vertical jets which dissipates wave energy. Under storm attacks, the geometry of reef balls reduces its stability whereas its porosity, through the reduction of lifting forces, causes a beneficial compensating effect.
Geometry
Layout
There are many schemes adopted for ARs, especially for those made by geotextile, depending on their principal own function. After defining the objectives to be achieved, an appropriate geometry can be selected. Primarily, we consider the case of submerged structures that are used to defend the coast from erosion. The layout in Figure 1.a highlights some similarities with the configuration of traditional rubble mound barriers parallel to the littoral with gaps in between. The waves that break over the reef induce an inshore increase of sea level and at the same time a decrease of wave height due to energy dissipation over the crest (Pilarczyk, 2003[6]). Figure 2 shows some sample layouts for Artificial Surfing Reefs(ASRs). The structure is composed by two convergent wedges and a sharp focus where the waves break (Ranasinghe et al., 2006), defining a-frame waves, i.e waves that break in a central point while breakpoint evolves in symmetrical way from
Cross-section
Dimensions
Function
See also
References
- ↑ Bohnsack, J.A., 1989. Are high densities of fishes at artificial reefs the result of habitat limitation or behavioral preference? Bulletin of Marine Science. 44, 631-645.
- ↑ Burcharth, H. F., Hawkins, S., Zanuttigh, B., and Lamberti, A., 2007. Environmental Design Guidelines for Low Crested Coastal Defence Structures, Elsevier.
- ↑ Matteotti G., Ruol P., 2003. L’impiego dei geosintetici nelle opere di ingegneria marittima e costiera, 2003 [1]
- ↑ O’Leary, Hubbard T., O’Leary D. Artificial Reefs Feasibility Study, Coastal Resources Centre National University of Ireland Cork, 2001
- ↑ Harris, L.E., 1995. Engineering design of artificial reefs. Oceans '95, Marine Technology Society, Washington, D.C., Vol. 2, pp. 1139-1148.
- ↑ 6.0 6.1 Pilarczyk, K.W., 2003. Alternative systems for coastal protection: an overview. International Conference on Estuaries and Coasts November 9-11, 2003, Hangzhou, China.
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